1. Technical Field
This invention relates to apparatus and methods for radio communication between a wireless unit and one or more static network access points. In some embodiments, the wireless unit may be an environmental or positioning sensor.
2. Background Information
There are many contexts in which it is desirable to transmit information wirelessly from a wireless unit onto a fixed network, rather than requiring a wired connection. For example, the wireless unit may be part of a positioning system, in which it is important for the unit to be mobile, e.g. when it is carried by or attached to a person or item of movable equipment. Alternatively, the wireless unit may be relatively static, but situated in a location to which it is undesirable or impossible to install a wired connection; e.g. a temperature sensor in a building.
Many environments, such as buildings, already contain networks with radio access points; commonly based on one or more of the Institute of Electrical and Electronics Engineers (IEEE) 802.11 standards (commonly known as ‘WiFi’). However, associating a wireless unit with such an access point can be slow and power-intensive for the wireless unit, especially when complex authentication protocols are employed. Maintaining association can place an additional power demand on the wireless unit, which may typically be powered by a relatively small battery.
WO 2006/120675 describes an asset transmitter that transmits a presence signal to a wireless local area network (WLAN) system in a broadcast or multicast format that is compatible with IEEE 802.11, but which does not require that the transmitter be associated with an access point. However, if it is desired to send commands to the asset transmitter, so as to control transmission of the presence signal and other parameters, the asset transmitter must include a WLAN receiver, which causes a substantial power drain on the asset transmitter. An attempt to mitigate this problem is described in US 2005/0156711 by only powering a receiver on an asset tag to receive commands during a receive window after each transmission by the asset transmitter. However this can still lead to unnecessarily high power consumption, since the receiver must be powered up even when there are no commands to be received. It also only allows commands to be sent to the asset tag during limited time windows, which may make it impossible to issue a control command in a timely manner. It may also be undesirable or impossible to implement such a proprietary system using standard wireless network infrastructure components.
The present invention seeks to overcome these shortcomings.